In various industries, various efforts to reduce environmental burden have been made on a global scale. In particular, in the automobile industry, the development of techniques has progressed for the spread of not only a gasoline engine vehicle having superior fuel efficiency but also a so-called eco car such as a hybrid vehicle or an electric vehicle and for further improvement in the performance of the vehicles.
Incidentally, in an exhaust system for exhaust gas that connects a vehicle engine and a muffler to each other, a catalytic converter for purifying exhaust gas is generally provided.
The engine may emit environmentally harmful materials such as CO, NOx, or unburned HC or VOC. In order to convert such harmful materials into environmentally acceptable materials, catalyst layers formed of a noble metal catalyst such as palladium or platinum are formed on cell wall surfaces of a substrate including plural cells. More specifically, on the cell wall surfaces of the plural cells, the catalyst layers are formed in a longitudinal direction of the substrate which is a direction in which exhaust gas flows. By causing exhaust gas to flow through the catalytic converter including the substrate having the above-described configuration, CO is converted into CO2, NOx is converted into N2 and O2, and VOC is burned to produce CO2 and H2O.
However, for example, for a substrate including cells having a honeycomb structure, a catalytic converter having a uniform cell density of the substrate is generally used. However, since an exhaust gas flow rate in a center region of a cross-section of the substrate is higher than that in a peripheral region thereof, there is a problem in that the catalyst layers of the entire substrate cannot be sufficiently utilized. Therefore, by using a catalytic converter in which a cell density of a center region of a substrate is higher than that of a peripheral region thereof in consideration of the above exhaust gas flow rate variation, a flow rate variation in a cross-section of a substrate can be reduced as much as possible. Accordingly, catalyst layers of the entire catalytic converter can be efficiently utilized to purify exhaust gas.
Here, PTL 1 discloses a technique of improving exhaust gas purification performance by causing the amount of a noble metal catalyst supported on a center region (here, middle portion) to be different from that supported on a peripheral region (here, peripheral portion), for example, in a substrate having a honeycomb structure in which the cell density is uniform in the entire catalytic converter (here, catalyst body). More specifically, in the catalyst body, the amount of catalyst per unit volume supported on the middle portion having a large amount of gas flow is set to be 1.1 times or higher than that on the peripheral portion. However, in this technique, the cell density of the entire catalyst is uniform, and thus it is difficult to expect high exhaust gas purification performance. In addition, this technique also has a problem in that, when the amount of a noble metal catalyst increases, a large amount of hydrogen sulfide which causes an odor during exhaust gas purification is produced.